One embodiment of the present invention is directed to thin film deposition on a substrate. More particularly, one embodiment of the present invention is directed to optical monitoring of thin film deposition on a substrate.
There is a growing demand for precision optics instruments based on optical thin films, particularly for telecommunication, surveillance, astrophysics, laser and printing technology, medical instrumentation. etc. This has resulted in a growing need of precise in-situ optical monitoring of the growing thin films in the process of deposition. The new generation of precision optical coatings require in-situ determination of a number of physical and optical properties of the deposited layers, such as optical thickness, complex index of refraction of the deposited layer and the thin film stack as a whole, inhomogeneity, absorption, scattering, etc.
Known in-situ optical monitoring systems typically operate by monitoring the light interference through the coated substrate or a special sample substrate (referred to as a “witness”), and by calculating the needed optical parameters of the system “substrate coating”, or by monitoring the polarized components of the light coming from the substrate and calculating its ellipsometric parameters. The known systems may monitor the transmitted light, the reflected light, or both.
In all known systems, however, the monitoring beam enters the deposition chamber through a transparent window mounted on the chamber walls, and leaves the chamber through another window in the case of transmission monitoring, or through the same or another window in the case of reflection monitoring. The needed clear path for the optical beam running through the chamber has to be provided for when the deposition chamber is designed. In some cases, a system of mirrors or beam splitters is used to direct the monitoring beam to the monitored sample or to take a power reference of the monitored light. In some other cases, a system of polarization components is also used to make ellipsometric measurements.
FIG. 1 is a sectional view of a prior art vacuum deposition chamber 10 having optical monitoring of a thin film deposition on a substrate. Vacuum deposition chamber 10 includes optical windows 11-14, and a dome substrate holder 15 for holding substrates 16 and a monitoring sample 18 (or witness 18). A mask 20 provides the needed uniformity of the coating over all of substrates 16. Chamber 10 further includes a mechanical shutter 22 that closes a vapor source 24 and prevents vapors 26 generated by an electron beam 28 to reach substrate 16 as the optical thickness on monitoring sample 18 is reached. The monitoring can be performed in transmission 30, in reflection 32 from the coated side of the sample 18 or in reflection 34 from the uncoated side of sample 18.
The optical monitoring of chamber 10 is performed on witness 18, which may be monitored constantly during the whole deposition process or may be changed a few times during the process. Witness 18 is placed approximately at the center of the rotation of substrate holder 15. Substrate holder 15 typically has a dome shape, but shapes such as plane, cylindrical or others are also possible. Planetary rotation of the substrates is also often used.
With a specially designed mask, the optical monitoring of witness 18 performed at one or several wavelengths can ensure that the right optical thickness of the layers is deposited on the samples positioned on a dome and performing one or more independent rotations with respect to the center of the dome. When the desired optical thickness of the layer is reached on witness 18, vapor source 24 closes automatically by shadowing the vapors with mechanical shutter 22. Typically the monitoring light beam is modulated at some frequency and lock-in amplifiers are used to detect the modulation frequency. The intensity of the passing through the witness light is measured at a certain wavelength. In some cases the incident light beam is split on two—one monitoring beam and one reference beam. Further, in other cases during one modulation cycle the same beam passes a first phase when it goes for monitoring of the substrate, a second phase when it goes for reference and a third “dark” phase.
FIG. 2 is a sectional view of another prior art vacuum deposition chamber 40 having optical monitoring of a thin film deposition on a substrate. Chamber 40 has optical windows 41, 42 for monitoring beam 43, and a rotational substrate holder 44 with a substrate 45. A mechanical shutter 46 c loses the vapor source 47 and prevents the vapors 48 generated by the electron beam 49 from reaching substrate 45 as the optical thickness on the coating is reached. Monitoring beam 43 operates in transmission.
In chamber 40, the optical monitoring is performed in transmission directly through the center or slightly off-center of the rotational substrate. The optical beam entering the chamber is first modulated with a certain frequency and focused on the monitored substrate. After passing through the substrate the beam is re-focused on the entrance slit of the monochromator and then the beam with a selected wavelength goes to the detector. In order to transport the light beam outside the chamber very often different optical fiber bundles are used. When the desired optical thickness of the layer is reached on the substrate, the vapor source closes automatically by the shutter. This arrangement allows direct monitoring of the deposited coating without need for a mask.
FIG. 3 is a sectional view of another prior art vacuum deposition chamber 50 having optical monitoring of a thin film deposition on a substrate. Vacuum deposition chamber 50 has optical windows 51, 52 for monitoring a beam 53, and a rotational substrate holder 54 with a substrate 55. A mechanical shutter 56 closes substrate 55 and prevents the sputtered particles 57, generated as a sputtering ion beam 58 from a sputtering ion source 59 and which sputters a target 60, to reach substrate 55 as the optical thickness on the coating is reached. Monitoring beam 53 operates in transmission.
In prior art chamber 50, the optical monitoring is performed in transmission slightly off-center of the rotational substrate through the windows mounted on the chamber walls. The deposition particles are sputtered from a target by means of sputtering source. When the desired optical thickness of the layer is reached on the substrate the shutter is closed automatically in front of the substrate. As with chamber 40 of FIG. 2, this allows direct monitoring of the deposited coating without the need of a mask, even though a mask can also be used. The monitoring is performed either during the full rotation cycle of the substrate or during a part of it.
In general, known optical monitoring systems have several significant drawbacks. They typically allow only one monitoring system per deposition chamber. This does not allow multiple optical monitoring systems to be used in the same chamber in order to monitor more than one or all deposited substrates at the same time. In addition, known optical monitoring systems cannot deposit optical coatings with different designs on different substrates at the same time. This results in inefficient use of the chamber space, the electrical power, the coated material and the deposition time.
Further, in known systems where the monitoring is performed through the deposited substrate there is only one deposited substrate at a time. No multiple substrate deposition is possible. In addition, all known systems have a deposition source mechanical shutter which automatically shuts the vapor source and stops the deposition on all the substrates at the same time. None of the systems allow for each substrate to have its own autonomous shutter.
Further, known monitoring systems are designed to fit a certain chamber geometry and cannot be used for other geometries without significant redesign, which is not always possible and is expensive. The chamber design includes the optical monitoring system installation. If the chamber is designed not to have optical monitoring system (as 80% of all the deposition chambers are) it is very difficult to retrofit an optical monitoring system unless significant hardware work is performed.
Further, none of the known systems are portable and designed to fit all chamber geometries. Most existing chambers, including old chamber models, are not designed to have optical monitoring system. These chambers have limited use and do not allow deposition of complex optical coatings and filters because of the lack of monitoring.
Based on the foregoing, there is a need for an optical monitoring system that is portable and adaptable to current chambers, and can monitor multiple substrates simultaneously.